Clouds of pollution and other airborne materials are difficult to detect. Yet the concentration of these materials needs to be measured to enable those near the cloud to respond in an appropriate manner. Without an accurate assessment of the strength of the cloud unnecessary evacuations may be ordered or a needed evacuation may not be deemed necessary. The materials that can occur in these clouds include man-made pollution (e.g. a release from a chemical plant), natural pollution (e.g. volcanic fumes), and chemical and biological warfare agents. The difficulty of detecting these clouds arise for several reasons. First, the materials may be invisible and otherwise undetectable by human beings even at concentration levels that pose an immediate health threat. Second, the clouds tend to move with the wind so that, once released, they can travel long distances, thereby appearing without warning. Further, drafts, inversions, and other thermal gradients can cause the cloud to concentrate in some geographic areas (e.g. valleys) while dispersing rapidly from other areas (e.g. hilltops). Similarly, the cloud might be found at some altitudes and not found at others. Also, because these clouds might be found at some height above the ground, it may not be possible to place an instrument in the cloud short of flying a probe into the cloud. The clouds may also have irregular shapes with ill-defined boundaries (i.e. the cloud boundary may be marked by either a sharp or gradual concentration gradient or some combination of the two). Thus, where a particular cloud might be found is subject to a number of vagaries that cause difficulties in predicting where the cloud might exist.